MODES OF INHERITANCE IN HYBRIDS 467 



3. SUMMARY AND DISCUSSION OF DATA SHOWN IN TABLES 



a. Successful crosses 



Out of twelve crosses possible between the four species dealt 

 with in this paper four have been successful in the sense that 

 development up to hatching took place and more or less viable 

 larvae were produced. 



The most successful crosses are those between F. diaphanus 

 and F. heteroclitus, in spite of the fact that the former is a 

 fresh water species and the latter a marine species. These two 

 species cross reciprocally with equal facility, producing in each 

 case some larvae that hatch earlier than those from pure bred 

 eggs of the maternal species. These early larvae are also, in 

 both reciprocal crosses, unusually vigorous and seem to excel 

 the larvae of either parent species in viability and rapidity of 

 growth. It is important to note here that F. heteroclitus devel- 

 ops somewhat more rapidly than F. diaphanus, but that the 

 developmental rates of both reciprocal crosses are more rapid 

 than those of the respective egg species, and that in the case 

 of the F. diaphanus egg hybrids this is very marked. These 

 facts are entirely out of accord with the observations and state- 

 ments of Moenkhaus, who claims that the invariable effect of 

 the foreign sperm in fish hybrids is to retard development, never 

 to accelerate it. He considers this universal retardation to be 

 the result of some injury to either the egg or sperm substances 

 of the zygote, akin to the well known hemolytic effects observed 

 in experiments dealing with the transfusion of foreign blood. 

 The hemolysis parallel fails to apply, however, in these cases and 

 in others where no retardation occurs but where a pronounced 

 acceleration is evident to any one who takes the trouble to make 

 carefully controlled studies of comparative rates of development 

 of pure and hybrid strains. In these crosses in which early accel- 

 eration has been observed it is conceivable that we are dealing 

 with effects akin to those rejuvenating or stimulating effects so 

 often noted when diverse strains of the same stock are crossed. 

 Certain new combinations of morphological and physiological 

 characters are more readily produced and occur more rapidly 



